If the most demanding game you play is WOW then I would say the 9800gtx would qualify as overkill.
I'd say spread that money around a bit, maybe upgrade the cpu, q6600, but definitely a new card like the 8800gt or 9600gt. Depending on your upgrade budget.
What's your mobo? might need to upgrade that as well if you go to a quad.
Put your current system specs in your sig so we can see what else you have that may cause an issue....